API Gateway

What it is

An API gateway is the entry point for client requests. It routes requests to backend services and often handles cross-cutting concerns such as authentication, rate limiting, request validation, API versioning, and response shaping.

Why it matters

It keeps every backend service from reimplementing the same public API rules. Product teams can change internal service boundaries while clients keep calling stable endpoints.

How it works

The gateway inspects method, path, headers, identity, and sometimes request body. It applies policy, chooses a backend route, forwards the request, and returns the response with standard headers and errors.

When to use it

Use it for public APIs, mobile backends, multi-service products, and platforms with different customer tiers or API versions.

Tradeoffs / failure modes

A gateway can become a bottleneck or a dumping ground for business logic. Keep it focused on platform policy, routing, and protocol translation.

Tiny example

POST /v1/chat/completions hits a gateway that authenticates the API key, checks token limits, and routes the request to a model-serving service.

Related guides and systems

See API Versioning, Rate Limiting, OAuth2 & Social Login, ChatGPT, Slack, GitHub, and Gmail.